Thin film disks (TFD) in magnetic direct access storage devices (DASD) or rigid disk files require very smooth surfaces and low fly heights of the transducer carrying slider to achieve the progressively higher densities that these devices are being designed to attain. The necessity that these devices be contained within a predetermined form factor mandates that optimum utilization be made of available space.
Air is exchanged between the enclosure surrounding the head disk assembly and the atmosphere of the surrounding environment by diffusion and flow. These storage devices normally exchange some air with the surrounding environment even when there is no internal recirculating air stream. The head-disk enclosure is normally vented to the atmosphere and internally pressure balanced to prevent excess pressure and leaks which may allow contaminated air to enter the file. To prevent contamination, files are equipped with a breather filter to remove 99.99 percent of particles from the external atmosphere entering the file. Files will heat up during operation due to energy dissipation from the spindle motor and actuator as well as from air friction with the spinning disks. During this thermal cycling there will be an air exchange or "thermal pumping". Also there may be small leaks at the connections, gaskets, screw holes and the motors, which dictate that a proper design provide a pressure balance to allow a small flow in through the breather and out through any leak points during power-on.
Air contaminants dangerous for magnetic disk drives include many types of vapors. Heavy organics and organometallics are examples of undesirable substances. They include hydrocarbons, phthalates, adipates, fatty amines and phenols. Corrosive inorganic gases such as hydrogen chloride, hydrogen sulfide, sulfur dioxide, nitrogen oxides and ammonia can injure components in a head-disk assembly (HDA). Thin film disks used in current higher density storage devices are more susceptible to corrosion than prior particulate disks. In general, all reactive or adsorbing gases and vapors are potentially hazardous.
One important failure mechanism of magnetic disk drives is stiction, or static friction, which is adhesion of the heads to the disks while the file is stopped. Due to size constraints, the space available for drive motors is small and the motors have limited torque. High densities being achievable only with very smooth magnetic layers and very low fly heights, it is necessary that both disk and slider be very smooth. Such smooth surfaces are very susceptible to stiction resulting from the accumulation of organic materials on the surfaces. The lubricants used in these devices are present in very thin layers (on the order of a monomolecular layer). This makes the lubricant layer very susceptible to contamination by small amounts of material adsorbed on the surface. These adsorbed materials change the surface energy and the adhesive forces between the disk and slider. Vapors may also undergo capillary condensation in the gap between slider and disk. There is no way to predict what chemicals will be in the environment of a disk file. This is especially true in small files destined for small to medium sized computer systems which may not be used in the typical data processing environment.
A breather filter is subject to various design considerations. The filter must remove external particulate contamination from the air entering the file. The filter must prevent the entry of high molecular weight contaminants into the file and if corrosive gas protection is required, the filter must be capable of removing corrosive gases.
The filter must also meet certain criteria to be useful in compact storage devices. It must meet the functional performance requirements for the life of the file. The filter must be capable of fitting into design requirements of low end storage devices in which space is at a premium and where there are significant limitations on the location of the breather filter. The filter must remove a very large fraction of the external contaminants entering the file. Depending on the leak and thermal pumping rate, the required efficiency may exceed 99.5 percent. The filter must be low cost and readily manufacturable.
To obtain maximum life from the filter while maintaining low cost and small size, it is essential the the adsorber element contacts only the air actually entering the file. Chemical vapors in the vicinity of the filter element can diffuse into the active adsorbers and reduce capacity. Consequently. maximum filter life at minimum cost and size will require that the adsorbers be limited in exposure to external contaminants contained in the actual flow into the file. This requires that the adsorbers be protected by diffusion elements.
The prior art includes a breather filter in the May 1988 issue of Research Disclosure which uses a compact design, but does not include both a particulate prefilter and final filter in combination with a chemical filter, nor teach equalized flow distribution. U.S. Pat. No. 4,684,510 shows a disk drive filter device that uses a diffusion tube to communicate with the exterior atmospheric environment and incorporates chemical filtering capability, but does not teach the equalized flow distribution through the filter medium that would maximize effectiveness and the useful life, nor does it teach features to allow ultra-low profile.